MERSENIUS.--With its extensive rill-system and interesting surroundings, one of the most notable ring-plains in the third quadrant. It is 41 miles in diameter, and is encircled by a fine rampart, which on the side fronting the Mare Humorum rises 7000 feet above the floor, which is distinctly convex, and is depressed 3000 feet below the region on the E., though it stands considerably above the level of the Mare. The prominently terraced border is tolerably regular on the N.W., but on the S. and S.E. is much broken by craters and depressions, the largest and most conspicuous interrupting the continuity of its summit-ridge on the latter side. A fine crater-row traverses the central part of the interior, nearly axially, and a delicate cleft crosses the N. half of the floor from the inner foot of the N.E. wall to a crater not far from the opposite side. I detected another cleft on November 11, 1883, also crossing the N. side of the floor. South of Mersenius is the fine ring-plain Mersenius _d_, about 20 miles in diameter, situated on the border of the Mare; and, extending in a line from this towards Vieta are two others (_a_, and Cavendish _d_,), somewhat larger, but otherwise similar; the more easterly being connected with Cavendish by a mountain arm. One of the principal clefts of the system (all of which run roughly parallel to the N.E. side of the Mare, and extend to the Percy Mountains E. of Gassendi) crosses the floor of _d_, and, I believe, partially cuts into its W. wall. Another, the coarsest, abuts on a mountain arm connecting _d_ with Mersenius, and, reappearing on the E. side, runs up to the N.W. wall of the other ring- plain, _a_, and, again reappearing on the E. of this, strikes across the rugged ground between _a_ and Cavendish _d_, traversing its floor and border, as does also another cleft to the N. of it. Cavendish _d_ includes a coarse cleft on its floor, running from N. to S., which I have frequently glimpsed with a 4 inch achromatic. There are two other delicate clefts running from the Gassendi region to the S.W. side of Mersenius, which are in part crater-rills. CAVENDISH.--A notable ring-plain, 32 miles in diameter, S.E. of Mersenius, with a prominently terraced border, rising at one point on the S. to a height of 6000 feet above the interior, on which are a few low ridges. A large bright ring-plain (_e_), about 12 miles in diameter, breaks the continuity of the S.E. wall, and adjoining this, but beyond the limits of the formation, is another smaller ring with a central hill. There is also a bright crater on the N.W. border. The W. _glacis_ is very broad, and includes two large shallow depressions. An especially fine valley runs up to the N. wall, to the W. side of _e_.